1. Field of the Invention
The present invention relates to a web conveying device, a printing apparatus, and a tension control method.
2. Description of the Related Art
In a printing apparatus for forming an image on a web (rolled sheet), there is a known technique for conveying a web having no feed hole by a roller conveyance mechanism to form an image on the web.
When a web does not have feed holes, accurate conveyance of the web to an image forming unit becomes more difficult as printing speed increases. For example, Japanese Patent Application Laid-open No. 2001-335206 discloses a technique to deal with such a situation.
Moreover, when a web does not have feed holes, a conveyance load at the time of the beginning of web conveyance fluctuates sharply as printing speed increases. Since such fluctuation in the conveyance load causes web conveyance speed to fluctuate, an image forming failure occurs when an image is formed on the web. For example, Japanese Patent Application Laid-open No. 2003-327356 discloses a technique for reducing image forming failures by suppressing fluctuation in the web conveyance speed by changing tension to be imparted to a web.
Moreover, a load applied to a web varies depending on the width of the web to be used. When a load applied to a web is small at the time of the beginning of web conveyance, application of a load in a direction other than a web conveyance direction deteriorates directionality of web conveyance, causing skew of the web and deterioration of printing quality. In addition, a conveyance load at the time of normal travelling increases as a width of a web to be used increases. Consequently, when a web has a wider width, a conveyance load fluctuates sharply, and thus an image forming failure is likely to occur.
For example, Japanese Patent Application Laid-open No. 2004-182457 discloses a technique in which a rotational position of a tension generating roller at the time of the beginning of web conveyance is determined by width and ream weight of a web to suppress fluctuation at the time of the beginning of web conveyance, so that travelling failures and image forming failures are reduced.
In addition, for example, Japanese Patent Application Laid-open No. 2006-248722 discloses a technique for imparting appropriate tension to a web to be conveyed when the web has a difficulty in receiving tension. According to the technique, the appropriate tension is imparted by a tension generating roller and a tension adjusting guide that operates separately from the tension generating roller.
Moreover, for example, Japanese Patent Application Laid-open No. 2004-250203 discloses a technique used in a web tension-applying device of a printing apparatus. In the web tension-applying device including a web tension changing unit for changing tension of a web by drums sandwiching the web and a pressing force of a pressing roller, and a buffer mechanism for visually checking strength of the web tension, the drums of the web tension changing unit and the buffer mechanism are integrally provided.
Moreover, in a case where tension is imparted to a web, appropriate tension data that satisfies conditions such as width, ream weight, smoothness, and water content rate of the web needs to be prepared for each web. However, such data preparation for each web causes an increase in data volume. For example, Japanese Patent Application Laid-open No. 2011-046489 discloses a technique for determining web conveyance resistance. According to the technique, a tension generating roller is rotated and fixed in a predetermined rotational position, and a position of a tension arm is detected, so that the web conveyance resistance is determined based on the detected positional information.
However, even when a rotational position of a tension generating roller at the time of the beginning of web conveyance is determined by width and ream weight of a web as similar to the above-described conventional technique, tension at the time of the beginning of web conveyance cannot be constant due to influence exerted by web variations and web changes. The web variations include a variation of web material or web glossiness and surface treatment in pre-printing. The web changes include a change in web surface smoothness due to print conditions such as duplex printing, and a change in a web state due to duplex printing.
Moreover, after a new type of web is printed, an operator needs to determine whether tension at the beginning of web conveyance is appropriate, and sets the appropriate tension at the time of printing the new type of web next time to start printing. This requires extra labor.
In addition, there are cases where appropriate tension is not set, for example, an operator has forgotten to set appropriate tension. If tension greater than target tension has been set, an images forming failure occurs. If tension less than target tension has been set, on the other hand, a trouble such as skew occurs due to reduction in web conveyance stability.
Moreover, in a case where short-time printing is repeatedly performed when tension is less than or greater than target tension, printing ends without knowing appropriate tension.
In addition, in a case where a tension generating roller is fixed to a predetermined rotational position, and web conveyance resistance is determined, web conveyance resistance determined using conveyance speed different from process speed such as automatic loading control and manual conveyance control may not be the most appropriate conveyance resistance for the process speed.
Moreover, in a case where a tension generating roller is fixed to a predetermined rotational position in the course of web conveyance at process speed, a trouble such as skew occurs due to reduction in web conveyance stability.
Therefore, there is a need for a web conveying device, a printing apparatus, and a tension control method that are capable of providing tension at the time of the beginning of web conveyance as appropriate tension.